The present invention relates to a blood oxygen saturation measuring apparatus which measures an oxygen saturation of a subject to be measured.
In order to detect a symptom of the sleep apnea syndrome, various related-art apparatuses and methods have been proposed. Among them, a related-art oxygen saturation measuring apparatus is known which obtains the oxygen saturation in arterial blood of the subject to estimate the respiratory motion of the subject, i.e., the variation in intrathoracic pressure (for example, see JP-A-10-216114).
As an oxygen saturation measuring apparatus, a pulse oximeter is used in which light beams of plural wavelengths that are different in light absorbency to the target substance such as oxygenated hemoglobin are transmitted through or reflected from living tissue, the intensity of reflected or transmitted light is continuously measured, and the oxygen saturation (SpO2) in arterial blood is obtained from a pulse wave data signal obtained in the measurement.
In the related-art oxygen saturation measuring apparatus disclosed in JP-A-10-216114, the respiratory status of the subject is monitored by measuring the oxygen saturation in arterial blood, and hence a time lag occurs between an obstructive apnea condition of the subject and a significant reduction of the oxygen saturation in arterial blood. In the case where the subject enters a central apnea condition during sleep, the reduction of the oxygen saturation in arterial blood is very gentle. Consequently, there is a problem in that, based on only the measured value of the oxygen saturation, it is difficult to determine whether the respiratory motion is normally performed or a central apnea condition occurs.
Furthermore, a related-art method has been proposed in which a respiration detector including a thermistor and a cannula sensor is attached between the nose and the upper lip to measure changes of the temperature and the pressure due to nasal air flow, and band-like strain gauges are wound around the chest and the abdomen to measure the phase difference and degrees of the motions of the chest and the abdomen, thereby detecting a central apnea condition. In the related-art method, however, the number of electrodes or the like to be attached is large. Therefore, for example, it is difficult to stably perform the respiratory management of a neonatal infant, because of error causes such as body motion.